Durable radially polatised tubular nanoreactors for catalysis

Rising energy demand and the need to reduce the use of fossil fuels to limit global warming have created an urgent need for clean energy collection technologies. One interesting solution is to use solar energy to produce fuels. Low-cost materials such as semiconductors have been the focus of numerous studies for photocatalytic reactions. Among them, 1D nanostructures are promising because of their interesting properties (high and accessible specific surface areas, confined environments, long-distance electron transport and facilitated charge separation). Imogolite, a natural hollow nanotubes clay, belongs to this category. Its particularity does not lies in its chemical composition (Al, O and Si) but in its intrinsic curvature, which induces a permanent polarization of the wall, effectively separating photo-induced charges. This nanotube belongs to a family sharing the same local structure with different curved morphologies (nanosphere and nanotile). In addition, several modifications of these materials are possible (coupling with metal nanoparticles, functionalization of the internal cavity), enabling their properties to be modulated. These materials are therefore good candidates as nanoreactors for photocatalytic reactions. So far, proof of concept (i.e. nanoreactor for photocatalytic reactions) has only been obtained for the nanotube form. The aim of this thesis is therefore to study the whole family (nanotube, nanosphere and nanotile, with various functionalizations) as nanoreactors for proton and CO2 reduction reactions triggered under illumination.

Compact source of electrons-positrons/muons-antimuons pairs

### Context
The context of this PhD thesis deals with laser plasma electron accelerators (LPA), which can be obtained by focusing a high-power laser into a gas medium. At focus, the laser field is so intense that it quasi-instantly ionizes matter into an undersense plasma, in which it can propagate. During laser propagation, the ponderomotive laser pressure expels plasma electrons from its path, forming a cavity void of electrons in its wake. This cavity, called ‘bubble’, can sustain accelerating fields (100GV/m) that are roughly three orders of magnitude larger than what can be provided by Radiofrequency cavities, which equip the current generation of conventional accelerators. These accelerating structures can trap some plasma electrons and accelerate them at relativistic energies (few GeVs) over distances of a few centimeters. This offers the prospect of producing much more compact and affordable accelerators, with the following goals: (i) democratizing their usage for existing applications currently reserved to only a few installations in the world (ii) enabling new applications in strategic sectors (fundamental research, industry, medicine, defense).

Among the applications for which a strong international competition exist we remark:

> The usage of these accelerators to provide the first high-energy (100 MeV) electron radiotherapy machine for medical treatmes

> The usage of these accelerators as a building block of a future large scale TeV electron/positron collider for high-energy physics

> The usage of these accelerators to develop a compact and mobile relativistic muon source to perform active muon tomography. Such a tool would be a major asset for industrial applications (e.g., safety diagnostic of nuclear reactors), and for defense applications (non-proliferation). It is worth to mention that in these two sectors the american agency DARPA has already funded an ambitious program ( Muons for Science and Security, MuS2) in 2022, with the aim of providing a first conceptual report of a relativistic moun source based on a plasma accelerator (cf. https://www.darpa.mil/news-events/2022-07-22).

### Challenges:

In order to enable the aforementioned applications, strong limitations of current laser-plasma accelerators need to be addressed. An important limitation is the low amount of charge at high-energies (100 MeV – few GeV) provided by these accelerators. The main reason behind the low accelerated charge is the fact that present-day injection techniques are based on the injection of electrons from the gas, whose density is very low. In order to address this limitation, we have recently proposed a new injection concept based on a remarkable physical system called “plasma-mirror”. This concept relies on the use of a hybrid solid-gas target. When impinging on such a target, the high-power laser fully ionizes the solid and the gas. The solid part is so dense that it can reflect the incident laser, forming a so-called ‘plasma mirror’. In the gas part, the laser propagates and drives a LPA. Upon reflection on the plasma mirror, ultra-dense electron bunches can be highly-precisely injected into the bubble of the LPA formed by the reflected laser field. As the solid offers orders of magnitude more charge than the gas medium and as charge is injected from a highly-localized region from the plasma (plane), it has the potential to level up the injected charge in LPAs while keeping a high electron beam quality.

The PHI group is an international leader in the study and control of these systems. In collaboration with LOA, by using a 100TW-class laser, we have demonstrated that this new concept allows for a significant increase of the accelerated charge while preserving the quality of the beam.

### Goals

The first objective of this PhD thesis will be to develop a multi-GeV laser-plasma accelerator based on a plasma-mirror injection on Petawatt-class laser installations like the APOLLON laser facility. With a Petawatt-class laser this accelerator should produce electrons beams at 4 GeV with a total charge of hundreds of pC and a few % energy spread. Such a beam quality would represent a substantial progress in the domain.

The second objective will be to send this electron beam into a high-Z converter in order to generate muons/anti-muons pairs. Our estimations show that we could obtain roughly 10^4 relativistic muons per shot, which would allow for the radiography of a high-Z material in a few minutes.

This PhD subject foresees:
> Theoretical/numerical modeling activities based on our exascale code WarpX (to model the laser-plasma accelerator) and on the Geant4 code (for the modeling of the high-Z converter).

> Experimental activities (high-intensity laser-plasma interaction, detection of relativistic muons)

The project involves several partner laboratories:

> The Laboratoire d’Optique Appliquée for the laser-plasma acceleration activities (A. Leblanc)

> The Lawrence Berkeley National Lab for code development activities (WarpX, J.L Vay)

> The CEA-IRFU for the detection part (micromegas technology, O. Limousin)

For the experimental part, we will use several laser facilities:

> The UHI100 laser installation for the setup and testing of the laser-plasma accelerator at reduced power

> The APOLLON installation for the setup and testing of the plasma accelerator with a PW-class laser. A first experience implementing the concept of a plasma-mirror injector at the PW-level is scheduled for May 2024 in the framework of a collaboration between CEA and LOA. Following this experiment, we will perform a second experiment (2025-2026) to generate muons on APOLLON or other laser facilities in Europe (e.g., the ELI installations).

Virtual neutron scattering experiments from the moderation to the neutron detection.

The French neutron scattering community is proposing to build a new High-Current Accelerator-driven Neutron Source (HiCANS). Such a source would use a low-energy proton accelerator, a few tens of MeV, to produce thermal and cold neutrons and power an instrumental suite of around ten spectrometers. The aim of the thesis project is to build a multi-scale description of the operation of a neutron scattering spectrometer, ranging from the description of microscopic neutron moderation processes and neutron interactions with atomic structure and sample dynamics, to the propagation of neutrons through advanced optical elements and the production of background by secondary particles. The ultimate aim is to be able to carry out virtual neutron scattering experiments and accurately predict instruments performances on the future ICONE source.

Impact of microstructure in uranium dioxide (UO2) on ballistic and electronic damage

During reactor irradiation, fuel pellets undergo a partial evolution of their microstructure. At high levels of burnup, a subdivision of grains into smaller grains in the peripheral areas of the fuel pellets - called high burn-up structure (HBS) - is observed. Similar changes also occur in the central regions of the pellets at elevated temperatures. These evolutions result from the combination of several factors, including the loss of energy from fission products. The effect of this damage could vary depending on the crystal orientation and grain size.
The main objective is therefore to understand how crystal orientation and grain size influence the damage caused by irradiation. Ion irradiation experiments will be conducted on single- and poly-crystalline UO2 samples at the JANNUS Saclay facility. In situ and ex situ characterizations using Raman and Rutherford backscattering (RBS-C) spectroscopy, transmission and scanning electron microscopy with Electron backscatter diffraction (EBSD) will be carried out.

Radiolysis and irradiation: coupled effects on the corrosion and hydrogen uptake kinetics of zirconium alloys in primary water of pressurised water nuclear reactors.

The materials that make up nuclear fuel assemblies, including zirconium alloys, are exposed to the reactor's primary environment, pressurised water at high temperature (around 150 bar, 300°C), neutron bombardment and ionising radiation (gamma, alpha in particular). The combination of these external factors leads to the degradation of materials through corrosion and the formation of irradiation defects. The latter have been shown to have a significant effect on the corrosion and hydrogen absorption kinetics of zirconium alloys. However, the impact of irradiation is material-dependent. The corrosion rate of Zircaloy-4 increases after irradiation of the metal and/or oxide, while the corrosion resistance of the M5 alloy is significantly improved under irradiation. The presence of niobium in the alloy undeniably plays a positive role under irradiation. However, its effect and the associated mechanism remain to be elucidated. Given the short lifetime of the transient radiolytic species and the interplay of the various phenomena mentioned above, in situ measurements are becoming mandatory. The PhD student will work on using and upgrading a unique existing device, co-developed with Framatome, to monitor online the effects of water radiolysis and irradiation on the corrosion and hydrogenation kinetics of various zirconium alloys in contact with a medium representative of the primary medium of pressurised water nuclear reactors. Understanding the mechanisms involved will enable a model to be developed and the first-order parameters to be determined. The thesis work will be promoted through publications and participation in - national and internation - conférences.

Operando Bragg coherent diffraction imaging to probe CO2 Reduction

The imperative to capture and convert CO2 into high value-added chemicals or fuels represents one of the most significant challenges in achieving a sustainable society. This reaction can be performed in the gas phase at high temperature but also electrochemically, at low temperature, not only mitigating the greenhouse effect, but also providing a way to store energy by transforming intermittent renewable electricity into high added value chemicals. This project aims to investigate the structural evolution of individual nanocrystals during CO2 reduction reactions. Using the unique capabilities of Bragg coherent X-ray imaging, we can dynamically map, in situ and operando, the three-dimensional changes in lattice deformation, strain, composition, and crystallographic defects of nano-crystallites, establishing a comprehensive experimental framework for structure-chemistry-performance relationships. The experiments will be conducted at ESRF, the European synchrotron facility located in Grenoble, in close proximity to CEA-Grenoble, within a leading international scientific environment. The project will be in collaboration with LEPMI (Laboratory of Electrochemistry and Physico-chemistry of Materials and Interface, Grenoble-France), which has expertise in electrocatalysis, materials science, and energy storage and conversion systems.

2D materials under irradiation for tomorrow's functionalities

In view of the challenges posed by global warming, some fundamental research is focusing on optimizing the properties of materials for gas capture (e.g. CO2), filtration, desalination or the conversion of water to H2 by photocatalysis. Two-dimensional materials (graphene, MoS2, hBN, etc.) nanostructured by ion irradiation have recently shown unique and original properties to improve the efficiency of these processes. The introduction of surface modifications to these materials can be used to tailor their properties to specific requirements. Irradiation by fast heavy ions, such as those produced on the GANIL facility, or by low-energy ions produced on CIMAP's PELIICAEN device, induces surface modifications on the nanometric scale.

In this thesis, we propose to gain a better understanding of the processes involved in ion-beam structuration and the modification of 2D material properties as a function of the influence of different irradiation parameters on the local radiation-induced modifications.

Catalysis using sustainaBle hOllow nanoreacTors wiTh radiaL pErmanent polarization

The combined demands of increasing energy production and the need to reduce fossil fuels to limit global warming have paved the way for an urgent need for clean energy harvesting technologies. One interesting solution is to use solar energy to produce fuels. Thus, low-cost materials such as semiconductors have been intensively studied for photocatalytic reactions. Among them, 1D nanostructures hold promise due to their interesting properties (high specific and accessible surfaces, confined environments, better charge separation). Imogolite, a natural hollow nanotube clay belongs to this category. Although it is not directly photoactive in the visible light range (high band gap), it exhibits a permanent wall polarization due to its intrinsic curvature. This property makes it a potentially useful co-photocatalyst for charge separation. Moreover, this nanotube belongs to a family sharing the same local structure with different curved morphologies (nanosphere and nanotile). In addition, several modifications of these materials are possible (wall doping with metals, coupling with metal nanoparticles, functionalization of the internal cavity) allowing tuning band gap. The proof of concept (i.e., photocatalytic nanoreactor) was only obtained for the nanotube form.

This phD project aims to study the whole family (nanotube, nanosphere, and nanotile, with various functionalizations) as nanoreactors for reduction reactions of protons and CO2 triggered under irradiation.

Quantum fragmented states in frustrated magnets

The last few decades of condensed matter research have seen the emergence of a rich new physics, based on the notion of "spin liquids". Interest in these new states of matter stems from the fact that they exhibit large-scale quantum entanglement, a property that is fundamental to quantum computation. By directly exploiting this notion of entanglement, a quantum computer would enable revolutionary approaches to certain classes of problems, compared with conventional computers.

The study of spin liquids is therefore a key technological issue, and the aim of this thesis project is to contribute to this fundamental research effort.

Exploration of the energy deposition dynamic on short time scale with laser-driven electron accelerator in the context of the Flash effect in radiotherapy

The objective of the thesis project is to analyze the physicochemical processes resulting from the extreme dose rates that can now be obtained in water with the ultra-short (fs) pulses of relativistic electrons produced by laser-plasma acceleration. Indeed, first measurements show that these processes are probably not equivalent to those obtained with longer pulses (µs) in the FLASH effect used in radiotherapy. To achieve this, we propose to analyze the dynamics of formation/recombination of the hydrated electron, an emblematic species of water radiolysis, to qualify and quantify the dose rate effect over increasingly shorter times. This will be done in three stages in support of the necessary and now accessible technological progress, to have a dose per pulse sufficient to directly detect the hydrated electron. First, with the existing UHI100 facility, using the scavenging of the hydrated electron by producing a stable species; then producing a less stable but detectable species in real time and increasing the repetition rate of the electron source. Finally, by using an innovative concept named a “hybrid target”, based on a plasma mirror as an electron injector coupled to a laser-plasma accelerator, delivering larger doses with a narrower energy spectrum, we will be able to develop pump-probe detection allowing access to the shortest times, and to the formation in clusters of ionization, of the hydrated electron and measuring its initial yield.